This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We have discovered a striking example of remodeling of the sphingolipid (SL) metabolic pathway in the trypanosomatid protozoan Leishmania. In most eukaryotes, SLs are critical membrane components and signaling molecules, but our previous studies showed that Leishmania lacking the enzyme serine palmitoyltransferase (spt2-) lacked SLs and grew relatively normally. Their membranes contained the expected microdomains, but the spt2- mutants were defective in stationary phase differentiation and virulence. Surprisingly, similar phenotypes were observed in a degradatory SL mutant lacking the enzyme sphingosine 1-phosphate lyase (spl-). This epistatic interaction suggested that a downstream metabolite other than SLs were responsible for the phenotypic abnormalities. We demonstrated that in Leishmania, unlike other organisms, the SL pathway has evolved to be the major route for ethanolamine (EtN) synthesis and that EtN supplementation completely reversed the phenotypic abnormalities in viability and differentiation exhibited by both mutants. Thus, Leishmania has undergone two major metabolic shifts. The first is to de-emphasize the roles of SLs themselves for growth and differentiation, and this might reflect a unique combination of abundant membrane lipids in this parasite. Freed from constraints imposed in other eukaryotes to maintain precise levels of critical sphingolipid-derived signaling molecules, Leishmania have also been able to remodel SL metabolism towards a role in bulk EtN synthesis.